In the search for improvements in the efficiency of buildings such as by increased thermal insulation and weatherstripping (the process of sealing openings such as doors, windows, and trunks from the elements), modern buildings have become more airtight and less well ventilated and thus require some form of energy-effective ventilation system. MVHR systems are energy recovery ventilation systems which use an air-to-air heat exchanger, or Heat Recovery Unit (HRU) which causes the inbound air flowing into the building to exchange heat with the outbound air flowing out of the building, thus providing fresh air inside the building whilst also saving energy by reducing the heating (or cooling) requirement for the inbound air necessary for the interior of the building to be maintained at a comfortable temperature. There are many different types of air-to-air heat exchanger available, some of which are also capable of transferring the humidity level of the exhaust (outbound) air to the intake (inbound) air. MVHR systems are becoming increasingly prevalent in new buildings and new build houses. Many MVHR systems take advantage of buildings which are designed with a pitched roof, i.e. a roof having at least two pitched slopes; often there is unused space under at least a part of the pitched roof, so that many of the elements of the MVHR system (the HRU, the fans, etc.) can conveniently be placed in this space, and the ducting leading to the inlet for inbound air and to the outlet for exhaust air can also fit into this space. Consequently the inlet and outlet are often located on the pitched slopes, usually on different, often opposite slopes.
One problem with conventional MVHR systems is that they function by exploiting the difference between ambient temperature outside the building and the desired temperature inside the building. Whilst the desired internal temperature varies little according to season (the normal range of inside air temperatures which are deemed comfortable for humans is between about 16° C. and about 22° C.), ambient temperature outside the building varies over a much greater range (in the United Kingdom between about −5° C. and about 30° C.). In recognition of this variability in ambient temperature, and that whatever the ambient temperature one side of a building (the side which generally faces north) is usually cooler than another (the side which generally faces south), it is known to be good practice for MVHR systems to be installed so that inbound air is drawn from one side of the building, whilst outbound air exhausts from another side of the building, usually the opposite side (UK building regulations say that there must be a minimum of 30 cm between inlet and outlet vents, but it would be poor practice to site the outlet so close to the inlet, particularly if both were on the same side of the building). This takes advantage of the fact that when one side of a building is in direct sunlight, the opposite side is in shade. A feature of MVHR systems is that they are essentially static, in that their essential elements are fixed in position (they form part of the building). This means that if the inbound air is drawn from the southerly side of the building, in winter this air is warmer than the air on the northerly side of the house, thus reducing the energy required to raise it to the required internal temperature; however, in summer this inbound air is much warmer than the desired internal temperature, and warmer than the air on the northerly side of the house, thus increasing the energy required to lower it to the required internal temperature. If the inbound air is drawn from the northerly side of the building, in summer this air is cooler than the air on the southerly side of the house, thus reducing the energy required to lower it to the required internal temperature; however, in winter this inbound air is much colder than the desired internal temperature, and colder than the air on the southerly side of the house, thus increasing the energy required to raise it to the required internal temperature. One solution to this might be to reverse the flows through the MVHR system according to season; however, MVHR systems are not designed for reverse flows. Within the building, air inlets and outlets are located so as to be most efficient at introducing fresh air and removing exhaust air and so as to minimise the noise of the fan(s) which drive the air flows, so reversing the air flows would adversely affect the efficiency of the system, and lead to increased noise. It is also common for the inbound air to pass through a filter system, to remove undesirable dust, pollen, etc. from the inbound air before it enters the building; reversing the flow of air would risk allowing inbound air to entrain these filtered particles and carry them inside the building to the discomfort of the occupants. Further, if the air flows were to be reversed then it would be necessary to provide a second filter system to remove undesirable particles from the inbound air flow when the flow is reversed.